1. Technical Field
The present invention relates in general to fabrication of a mechanical structure on a substrate surface for robustly mounting Micro Electro Mechanical Systems (MEMS). More particularly, the present invention relates to a structure provided on a substrate surface for robustly mounting spring probes used on probe cards for testing integrated circuits on wafers.
2. Related Art
Mechanical structures are fabricated on substrate surfaces for a variety of applications. The substrate surfaces include polymers, metals, ceramics, semiconductors, etc. Typically to make electrical contact, a metal seed layer is deposited on the substrate surface to form bond pads for mounting the mechanical structures. As the size of the bond pad shrinks to accommodate smaller structures such as MEMS, or to accommodate limited spacing between small structures in an array, peeling or breaking of the metal seed layer from the substrate is more likely to occur, particularly with significant forces applied to mechanical structures formed on the bond pad. It, thus, becomes desirable to provide a robust support structure to prevent the metal seed layer from separating from the substrate.
One example of a MEMS structure mounted on a substrate surface includes a spring probe used to form a probe card for testing components on wafers, as illustrated in FIG. 1. A typical spring probe 2 is shown in FIG. 1 attached to a metal seed layer 4 provided on top of a layer 6 on a substrate 8. The layer 6 is optional, since the metal seed layer 4 can be directly connected to the substrate 8. The layer 6 is typically an insulator, but may be conducting, while the seed layer 4 is a metal, creating a metal-dielectric layer over the substrate 8. Metals making up the seed layer 4 can include copper, gold, silver, palladium, tungsten, titanium, aluminum, nickel, or a material that facilitates bonding of the spring probe 2, or further electroplating to form the spring probe 2. Alternatively, a bi-layer, multi-layer, or alloy of two or more of these materials, such as titanium-tungsten, copper-nickel-gold, etc. may provide the seed layer 4. Insulators making up layer 6 can include polyimide (PI), benzocyclobutene (BCB), FR4, ceramics, filled polymer or other materials. The substrate 8 is typically a multi-layer ceramic material, but may be a multilayer organic, metal matrix, metal, semiconductor or other. The spring probe 2 is typically composed of thin gold wire 10 surrounded by a resilient material 12, such as nickel cobalt, with a thin layer of gold plating 14 applied to maximize electrical conductivity. The fabrication of the spring probes 2 on the layer 6 involves application of the metal seed layer 4 on the layer 6 and then subsequently bonding and patterning the wire 10, and plating the wire 10 to form layers 12 and 14. The complete probe 2 is about the diameter of a human hair. Further details of spring probes used for wafer testing are described in U.S. Pat. No. 5,476,211 entitled “Method of Manufacturing Electrical Contacts Using A Sacrificial Member,” and U.S. Pat. No. 5,994,152 entitled “Fabricating Interconnects and Tips Using Sacrificial Substrates,” both incorporated herein by reference. Other types of spring probes similar to the spring probe of FIG. 2, such as those described in U.S. Pat. No. 6,255,126 entitled “Lithographic Contact Elements,” incorporated by reference herein, can likewise be attached to metal seed layers provided on a substrate surface. Although spring probes are referenced, other mechanical structures can be mounted on substrates, and may benefit from features of the present invention described subsequently.
Increases in the density of integrated circuits (ICs) on a wafer require a similar decrease in spacing between spring probes in an array used on a probe card for testing the ICs. A decrease in the spacing between spring probes in an array means a decrease in the size of the bond pads formed in the metal seed layer. As the bond pads shrink, the absolute breaking or fracture strength of the seed layer to its underlying substrate material becomes critical.
FIG. 2 illustrates how a force F applied to the probe springs during testing of ICs on a wafer can cause the metal seed layer pads to detach from the polyimide surface. As shown, the force F applied to the spring probe 2 during testing can cause the metal seed layer pad 4 to detach from the substrate 6 in area 16.
The mechanical robustness of structures formed on a seed layer, such as spring probes, depends on: (1) the size of the contact area between the seed layer and the substrate surface, (2) surface preparation of the substrate, and (3) the degree of adhesion between the seed layer and the substrate surface on which it is formed. Minor changes in processing conditions such as hydration/dehydration conditions can cause deterioration of the adhesion strength between the seed layer and the substrate, leading to catastrophic failure of the seed layer connection to the substrate surface. In some cases, robust parts cannot be fabricated as the peel strength required for such robust structures exceeds the adhesion strength of the seed layer to the substrate on which it is formed.
It would be desirable to provide a robust mechanical structure to prevent small bond pads, or mechanical structures formed on a substrate surface from detaching from the substrate surface when forces are applied to structures on the substrate.